docs(backend): refresh SSE endpoint guidance

[bamiebot-maker]

Summary

• restore and refresh the focused SSE README
• replace deprecated unversioned examples with authenticated /v1/events/subscribe and /v1/streams examples
• document the JWT requirement and 410 Gone behavior of the old unversioned subscription path
• mark JWT, per-IP limits, and Redis pub/sub as shipped with direct implementation links while retaining outstanding production TODOs

Closes #848

Validation

• verified every linked implementation file exists
• checked that no deprecated curl or stream endpoint examples remain
• git diff --check passes

Docs only; docs/SSE_ARCHITECTURE.md was not changed.

[bamiebot-maker]

CI audit: the docs-specific validation and both CodeQL jobs pass. The remaining matrix failures occur before or outside this README change: backend/frontend/Docker dependency installation fails on the existing Vitest 3.2.7 vs @vitest/coverage-v8 2.1.9 peer conflict, and contract jobs fail in existing Rust dependencies/tests. This PR changes only �ackend/SSE_README.md.